DC1572A - Demo Manual

DEMO CIRCUIT 1572A
LTC3615
QUICK START
GUIDE
LTC3615
DUAL 4 MHz, 3A SYNCHRONOUS
STEP-DOWN DC/DC CONVERTER
DESCRIPTION
Demonstration circuit 1572 is a dual output regulator
consisting of two constant-frequency step-down converters, based on the LTC3615 dual channel monolithic synchronous buck regulator. The DC1572 has an input voltage range of 2.25V to 5.5V, with each regulator capable of
delivering up to 3A of output current. The DC1572 can
operate in Burst Mode™, pulse-skip or forced continuous
mode. In shutdown, the DC1572 requires less than 100
uA total. The DC1572 is very efficient: over 90% for ei-
ther circuit. The LTC3615 comes in a 24 Pin QFN or
leaded package, which each having an exposed pad on
the bottom-side of the IC for better thermal performance.
These features, plus a programmable operating frequency
range from 400 kHz to 4 MHz (2.25 MHz switching frequency with the RT pin connected to SVIN), make the
DC1572 demo board an ideal circuit for use industrial or
distributed power applications. Gerber files for this circuit are available. Call the LTC Factory.
QUICK START PROCEDURE
The DC1572 is easy to set up to evaluate the performance of the LTC3615. For a proper measurement equipment configuration, set up the circuit according to the diagram in Fig. 1.
NOTE: When measuring the input or output voltage
ripple, care must be taken to avoid a long ground
lead on the oscilloscope probe. Measure the input or
output voltage ripple by touching the probe tip directly across the Vin or Vout and GND terminals. See
the proper scope probe technique in figure 2.
Please follow the procedure outlined below for
proper operation.
1. Connect the input power supply to the Vin and
GND terminals. Connect the loads between the Vout
and GND terminals. Refer to figure 1 for the proper
measurement equipment setup.
Before proceeding to operation, insert jumper
shunts XJ1 and XJ2 into the OFF positions of head°
ers J1 and J2, shunt XJ5 into the 180 out-of-phase
position of PHASE header J5, shunts XJ3 and XJ4
into the soft-start (EXT SS) positions of headers J3
and J4, shunt XJ7 into the forced continuous mode
(FCM) position of MODE header JP7, shunt XJ6 into
the 2.25 MHz (RT) position of the frequency
(RT/SYNC) header JP6, shunts XJCOMP1 and
XJCOMP2 into the external compensation (ECMP)
positions of headers JITH1 and JITH2, and shunt
XJP1 into the Vout1 voltage option of choice of
header JP1-4: 1.8V, 2.5V, 3.3V, or user select, and
shunt XJP5 into the Vout2 voltage option of choice
of header JP5-8: 1.2V, 1.5V, 1.8V, or user select.
2. Apply 5.5V at Vin. Measure both Vouts; they
should read 0V. If desired, one can measure the
shutdown supply current at this point. The supply
current will be less than 100 uA in shutdown.
3. Turn on Vout1 and Vout2 by shifting shunts XJ1
and XJ2 from the OFF positions to the ON positions.
Both output voltages should be within a tolerance of
+/- 2%.
4. Vary the input voltage from the min. Vin (which
is dependent on Vout) to 5.5V, and the load currents
from 0 to 3A. Both output voltages should be within
+/- 4% tolerance.
1
LTC3615
5.
Set the load current of both outputs to 3A and the input voltage to 5.5V, and then measure each output
ripple voltage (refer to figure 2 for proper measurement technique); they should each measure less than
30 mVAC. Also, observe the voltage waveform at either switch node of each regulator. The switching frequencies should be between 1.85 MHz and 2.65 MHz (T = 540 ns and 374 ns). In all cases, both switch
°
node waveforms should be rectangular in shape, and 180 out-of-phase with each other. Change the
shunt position on header J5 to set the switch waveforms in phase with respect to each other. To operate
the ckt.s in Burst Mode™ or pulse-skip mode, change the shunt in header J7. When finished, insert
shunts XJ1 and XJ2 to the OFF position(s) and disconnect the power.
Warning - If the power for the demo board is carried in long leads, the input voltage at the part could “ring”, which could affect the
operation of the circuit or even exceed the maximum voltage rating of the IC. To eliminate the ringing, a small tantalum capacitor (for
instance, AVX part # TPSY226M035R0200) is inserted on the pads between the input power and return terminals on the bottom of the
demo board. The (greater) ESR of the tantalum will dampen the (possible) ringing voltage due to the use of long input leads. On a
normal, typical PCB, with short traces, this capacitor is not needed.
Table 1. Performance Summary (TA = 25°C)
PARAMETER
CONDITIONS
VALUE
Minimum Input Voltage
2.25V
Maximum Input Voltage
5.5V
Run
Output Voltage VOUT1
Typical Output Ripple VOUT1
Output Regulation VOUT1
Output Voltage VOUT2
Typical Output Ripple VOUT2
Output Regulation VOUT2
Nominal Switching Frequencies
Burst Mode™ Operation
Pulse-Skip Operation
Phase
2
RUN Pin = GND
Shutdown
RUN Pin = VIN
Operating
VIN = 2.5V to 5.5V, IOUT1 = 0A to 3A
VIN = 3.3V to 5.5V, IOUT1 = 0A to 3A
1.8V ±4% (1.728V – 1.872V)
2.5V ±4% (2.4V – 2.6V)
VIN = 3.9V to 5.5V, IOUT1 = 0A to 3A
3.3V ±4% (3.168V – 3.432V)
VIN = 5V, IOUT1 = 3A (20 MHz BW)
Line
Load
< 30mVP–P
VIN = 2.5V to 5.5V, IOUT2 = 0A to 3A
±1%
±1%
1.2V ±4% (1.152V – 1.248V)
1.5V ±4% (1.44V - 1.56V)
1.8V ±4% (1.728V – 1.872V)
VIN = 5V, IOUT2 = 3A (20 MHz BW)
< 30mVP–P
Line
Load
RT Pin connected to 178k
RT Pin = SVIN
Channel 1: VIN = 5V, Vout1 = 3.3V
Channel 2: VIN = 5V, Vout2 = 1.8V
Channel 1: VIN = 5V, Vout1 = 3.3V
Channel 2: VIN = 5V, Vout2 = 1.8V
Phase Pin = SVIN
Phase Pin OPEN
Phase Pin = GND
±1%
±1%
2.25 MHz
2.25 MHz
Iout1 = 600 mA
Iout2 = 700 mA
Iout1 = 600 mA
Iout2 = 700 mA
180°Out-of-Phase
90°Out-of-Phase
In Phase
VIN = 2.25V to 5.5V, IOUT2 = 0A to 3A
LTC3615
Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple
3
LTC3615
Figure 3. VOUT1 Load Step Response
VIN = 5V, VOUT1 = 3.3V, 3A Load Step
Forced Continuous Mode FSW = 2.25 MHz
External Compensation: Rith = 43k, Cith = 220 pF
Trace 3: Output Voltage (100mV/div AC)
Trace 4: Output Current (1A/div)
4
LTC3615
Figure 4. VOUT2 Load Step Response
VIN = 3.3V, VOUT2 = 1.8V, 3A Load Step
Forced Continuous Mode FSW = 2.25 MHz
External Compensation: Rith = 43k, Cith = 220 pF
Trace 3: Output Voltage (100mV/div AC)
Trace 4: Output Current (1A/div)
5
LTC3615
Figure 5. Start-Up Operation
VIN = 5V, VOUT1 = 3.3V, VOUT2 = 1.8V, 3A Loads
VRUN = 2.5V
Forced Continuous Mode FSW = 2.25 MHz
External Compensation: Rith = 43k, Cith = 220 pF
Trace 3: Output Voltage (100mV/div AC)
Trace 4: Output Current (1A/div)
6
LTC3615
94
Vin = 5V
fsw = 2.25 MHz
Burst Mode
L1, L2 = 0.47 uH
2020BZ Vishay
92
Efficiency (%)
90
Vo1 = 3.3V
88
86
84
Vo2 = 1.8V
82
80
0.00E+00
5.00E-01
1.00E+00
1.50E+00
2.00E+00
2.50E+00
3.00E+00
Load Current (A)
Figure 6. LTC3615 DC1572 Efficiency
7
LTC3615
8
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